This invention relates to a method of recording and erasing signals by emitting high density energy flux on a rewritable optical information recording medium, and more particularly to a method of overwriting signals.
Several ideas have been proposed as a method of overwriting an information signal by emitting a laser beam onto a rotating optical information recording medium having a phase change type recording material such as chalcogenide glass film, that is, a method of recording new signals directly over the existing signals while erasing the existing signals. For example, a method of overwriting by using a single laser spot is disclosed in the Japanese Patent Laid-Open No. 56-145530. Furthermore a method of overwriting using two laser spots is unveiled in the Proceedings of SPIE, volume 420, p. 173.
The former method (the method of overwriting using a single laser spot) is generally known as the single laser beam overwrite method, in which the recording medium is irradiated with a laser beam modulated between two levels, a recording level and erasing level. At this time, the portion irradiated with a high laser power is once melted and then cooled to be amorphous, while the portion irradiated with a lower laser power is kept at a temperature exceeding the crystallization temperature for a specified time to be crystallized. When this process is realized, regardless of the state before irradiation with laser beam, that is, whether it is a recording mark part in an amorphous state or an erased part in a crystalline state, it is possible to overwrite using a single laser spot.
The single laser spot overwriting function has various merits, that is, the optical system is simplified, and the access time for rewriting is shortened to half (provided the rotating speed is identical). On the other hand, as a demerit, the erasing ratio is not sufficient (Japanese Journal of Applied Physics, vol. 26, 1987, Suppl. 26-4).
The other method (the method of overwriting using two laser spots) was the first demonstration of the image recording using a phase change type medium. The preceding light spot is elliptical, elongated in the track direction. When this spot passes over the medium the temperature of the medium is raised over the crystallization temperature, and it is kept in that state for a while to be crystallized. As a result, the recording signal recorded in an amorphous state is erased. The succeeding spot is circular. When this spot passes over the medium, the temperature of the medium rises rapidly, instantly exceeding the melting point, and it is quickly cooled the next moment. As a result, a record mark in an amorphous state is formed.
This two-spot method is advantageous in that a high speed overwrite was possible even if a recording material slow in the crystallization speed is used. In this method too, however, the erasing ratio was poor.
A method for improving the erasing ratio has been already disclosed, for example, in the U.S. Pat. No. 4,710,911. This is a method of realizing a melt-anneal process on a medium by disposing two light spots closely on a same track. The part irradiated with the preceding spot is instantly melted and the atomic distribution of the part is randomized to cancel its past history. Thus, a high erasing ratio is established. The succeeding spot irradiates with a moderate power so as not to melt the irradiated part, and crystallizes the portion after the above melting process. This is an operation for writing a new signal in the next step. In this method, however, a third spot is needed for recording a new signal during one pass, and the structure of the optical head has been complicated in order to focus the three spots on the same track at a high precision.